Air-to-air Exchanger?

[Bunkroom]

Over in the thread about floor material we got to discussing moisture control in the interior. I am wondering if anyone has installed an air-to-air exchanger in their airstream?

I found one 12V RV model online:

Air-Port Fresh Air Exchanger Air-Port Fresh Air Exchanger [771001] - $375.00 : UVS, Parts Store

I new homes, these are required and act to help manage interior humidity levels and to some degree temperature (depending on the outside-inside temperature differential).

Airstreams are not vapor permeable given that they are made out of aluminum and plywood. When we seal them well to keep liquid water out, we hold water vapor in.

It could likely be ducted (intake and output) through the rear or side of the trailer rather than through the roof to help avoid more spots for leaks. Interior ducting could be connected to existing where it exists or simple small flex could be added.

This could be run while underway to keep interior temps down on hot days. Also could be used as an alternative to a dehumidifier or running a/c under certain temp and humidity conditions.

The nice thing about air-to-air exchange over opening a vent and a window is that it will preserve some of your heat when its cold outside.

[cameront120]

A simple solution would be to turn on the bathroom and over-the-range exhaust fans. Many modern houses with no HVAC rely on a timer switched bathroom exhaust to exchange the air.

[c_lewis77]

I have installed these in homes. they are great for cycling (changing) the air in interior spaces. The trick that I have found is that in the home version, the point in which the unit is tied into the AC system is critical to avoid dumping untreated (humid) air back into the space. I always tie in the supply before the coils of the AC unit to allow for the dehumidification to occur. I also tie-in the Air exchanger to the A/C unit via switch so the Air Exchanger never runs without the A/c unit. This is especially improtant in homes that are sealed tight and do not allow "leaking" through the typical areas.
I would expect in this case that the air exchanger would be performing the same task as the supply/exhaust fans on the airstream but providing a filtering function as the added benefit. The scale of the RV probably diminishes the potential for this type of unit as well. Fantastic fans (one in intake, the other in exhaust) may accomplish the same thing without the ducting requirement. This is my opinion, however. I see that they are marketing this specifically for RV's. It would be interesting to find more technical information on this product.

[Gene]

An air exchanger in Ga. has different requirements than one in a cold climate. An RV one would need to be adaptable to a variety of conditions.

I expect the RV one is designed for MH's and may be too large for our trailers, but I haven't checked size.

We dissipate humid air by using bathroom and shower fans, but that wastes cool or hot air according to the season and isn't much help when it is humid outside. I think an air exchanger is a great idea and I hope someone buys one, figures out how to install it in a Safari just like ours, tests it, and reports back.

Gene

[danlehosky]

In an air exchange there will always be an atmospheric concequence. Even a very small differential will pull. I just don't know if this application would be good on an AS with all of the potential for water penetration. Maybe if you want to camp in cold climates you'll just have to accept the fact that you will need to vent stale moist air through the bathroom or stove vents and deal with it.

You could create a positive atmosphere with outside air that is heated but come on guys........These are campers.

Dan

[Bunkroom]

Originally Posted by c_lewis77

I always tie in the supply before the coils of the AC unit to allow for the dehumidification to occur. I also tie-in the Air exchanger to the A/C unit via switch so the Air Exchanger never runs without the A/c unit.
I would expect in this case that the air exchanger would be performing the same task as the supply/exhaust fans on the airstream but providing a filtering function as the added benefit.

Tying the intake to the upstream side of your cold coil is pretty inefficient! Hot outside air gets cooled (lets say to 40F) and dehumidified (this is good but will happen after the first pass anyway) before going to the air exchanger where it will get warmed back up to room temperature (say 72F or so) in the core by outgoing stale air. If you allow it to go through the core first, it will be cooled to close to room temp by the core and then further cooled when it ultimately goes through your air handler.

Either way, I think the real advantage of air-to-air exchange in an RV is during times when A/C is not needed or is not available (12V only).

The big advantage air-to-air exchange offers over simply running exhaust fans is the counter current exchange of temperature in the core. The air does not mix in the core, but heat from inside air is transferred to outside air warming it on the way in. This can be 60% efficient significantly reducing your heating fuel expenditure. This is shown well in the image you attached.



Let's assume its 70 in your trailer and 40 outside. You are breathing and cooking and increasing the interior humidity in the Airstream. The air exchanger core allows your interior warm air to exchange some of the heat in it with the cold outside air without mixing them. This can be 60% efficient, significantly reducing your heating fuel expenditure. You also have an intake source of fresh outside air other than the cracks and gaps in your trailer (which are hopefully minimal). So, its more than just a filter.

This doesn't work as well in the opposite situation where it is 90 and high humidity outside and you want it to be 70 and low humidity inside. For this situation A/C (or a dehumidifier) would be needed in which case the A/C unit will dehumidify for you. The air exchanger could still be used to bring in fresh air and expel stale indoor air.

[danlehosky]

Bunk, read my post on the floor thread.

[c_lewis77]

Originally Posted by Bunkroom

Tying the intake to the upstream side of your cold coil is pretty inefficient! Hot outside air gets cooled (lets say to 40F) and dehumidified (this is good but will happen after the first pass anyway) before going to the air exchanger where it will get warmed back up to room temperature (say 72F or so) in the core by outgoing stale air. If you allow it to go through the core first, it will be cooled to close to room temp by the core and then further cooled when it ultimately goes through your air handler.

Either way, I think the real advantage of air-to-air exchange in an RV is during times when A/C is not needed or is not available (12V only).

The big advantage air-to-air exchange offers over simply running exhaust fans is the counter current exchange of temperature in the core. The air does not mix in the core, but heat from inside air is transferred to outside air warming it on the way in. This can be 60% efficient significantly reducing your heating fuel expenditure. This is shown well in the image you attached.

I suppose i did not describe it very well. You are correct, the outside air goes through the core first then to the AC duct.
Yes in GA, these types of systems can work differently because we have high heat/humidity for a good portion of the year. In Minnesota there may be greater inefficiencies. I have seen these units work very well in a house. I am with Gene, it would be interesting to see how they perform in an RV!

- Casey

[Zilpha]

Great thread, thanks!!

p.s. @Dan, hi there... for you "these are campers"--but for some of us, these are homes...or we want/need to make our "camping" air as clean as possible.

[Gene]

Thoughts on heat exchangers:

An air exchanger is usually called a heat exchanger—outside air is brought in and heated in the winter before it is released inside. Warm air from inside is used to warm the intake air. In the summer, cooler interior air cools the outside air before it is released inside. Thus a heat exchanger can work both ways. Bunkroom's diagram shows how they work, but leaves out the relative complexity in practice of heat exchangers.

These are now used in some super sealed houses where the outside air doesn't get into the house naturally because all the numerous leaks in the house are sealed. Thus, the interior gets polluted by cooking and all the other things in a house. Thus a heat exchanger is installed in a wall to clean the interior air. These heat exchangers are not cheap. I have thought about installing one in a new house we bought, but have been put off by the cost. The cheap solution is to run kitchen or bathroom fans, thus creating a pressure drop in the house. Then, outside air comes in because of the pressure differential to the house through places where the cracks and leaks have not been sealed properly. This is a crude way of getting clean air into the house; the trailer operates the same way as interior fans are used to clean out pollutants and water vapor.

A forced air furnace has a heat exchanger built in—the fire box puts off nasty results of burning fuel, so it has to be isolated from the interior—the flame heats a separate part of the exchanger where interior air is circulated and heated. In any furnace, the polluted air is sent outside by a chimney or a side vent. Eventually a furnace heat exchanger fails—the seals shrink and crack or metal parts fatigue and let polluted air inside. Then it is time to fix or replace.

A trailer, being a small house, generates lots of polluted air and water vapor—the stove fan and bathroom and shower fans help get rid of some of it, but then you are sending heated sir outside when it is cold out. Same thing, except you are venting cooled air in the summer when you are running the A/C.

To solve the trailer problem would require a heat exchanger and a dehumidifier. These would have to made very small for a trailer and run on 12 v. Fans use a lot of energy in a trailer, thus requiring more batteries and/or solar panels. Miniaturizing these two would be expensive until you sold tens of thousands of them. You would have to vent each to the outside meaning you would have to find space for them along the exterior walls. Both would be most appreciated by those full timing and they may want to build such units and spend the money to create and install them. One result would be the loss of limited storage space. You would save some money because you would be using less energy on heat and A/C, but the net cost in initial investment and running them would probably not pay off for a long time.

It would be a worthy project for those who know how to build such things because anything on the market would probably be too big for a trailer. For the inventor who wants to create a product for sale, I'm not sure there would be much of a market for it at a reasonable price.

Gene